Dyes that change color over a range of temperatures are known as thermochromic dyes. Thermochromic dyes can be manufactured to have a color change that is reversible or irreversible. Formulated as pigments or colorants, they are used in a variety of applications such as plastic masterbatch, paper, textiles, coatings, offset ink, metal decorating inks, coatings, ultraviolet radiation curable inks and coatings, solvent based inks and coatings, screen inks and coatings, gravure inks and coatings, paints, security printing, brand protection, smart packaging, marketing and novelty printing, among other uses.
Thermochromic dyes use colorants that are either liquid crystals or leuco dyes. Liquid crystals are used less frequently than leuco dyes because they are very difficult to work with and require highly specialized printing and handling techniques.
Thermochromic dyes are a system of interacting parts. The parts of the system are leuco dyes acting as colorants, weak organic acids acting as color developers and solvents that variably interact with components of the system according to the temperature of the system. Thermochromic dye systems are microencapsulated in a protective coating to protect the contents from undesired effects from the environment. Each microcapsule is self-contained, having all of the components of the entire system required to reproduce the color change. The components of the system interact with one another differently at different temperatures. Generally, the system is ordered and colored below a temperature corresponding to the full color point. The system begins to lose its color at a temperature corresponding to a predetermined activation temperature.
Below the activation temperature, the system is colored and above the activation temperature they are clear or lightly colored. The activation temperature corresponds to a range of temperatures at which the transition is taking place between the full color point and the clearing point. Generally, the activation temperature is defined as the temperature at which the human eye can perceive that the system is starting to lose color, or alternatively, starting to gain color. Presently, thermochromic systems are designed to have activation temperatures over a broad range, from about −20° C. to about 80° C. or higher. With heating, the system becomes increasingly unordered and continues to lose its color until it reaches a level of disorder at a temperature corresponding to a clearing point. At the clearing point, the system lacks any recognizable color.
Specific thermochromic ink formulations are known in the art. See, for example, U.S. Pat. Nos. 4,720,301, 5,219,625 5,558,700, 5,591,255, 5,997,849, 6,139,779, 6,494,950 and 7,494,537, all of which are expressly incorporated herein by reference to the same extent as though fully replicated herein. These thermochromic inks are known to use various components in their formulations, and are generally reversible in their color change. Thermochromic inks are available in various colors, with various activation temperatures, clearing points and full color points. Thermochromic inks may be printed by offset litho, dry offset, letterpress, gravure, flexo and screen processes, amongst others. Thermochromic inks containing leuco dyes are available for all major ink types such as water-based, ultraviolet cured and epoxy. The properties of these inks differ from process inks. For example, most thermochromic inks contain the thermochromic systems as microcapsules, which are not inert and insoluble as are ordinary process pigments. The size of the microcapsules containing the thermochromic systems ranges typically between 3-5 μm which is more than 10-times larger than typical pigment particles as found in most inks. The post-print functionality of thermochromic inks can be adversely affected by ultraviolet light, temperatures in excess of 140° C. and aggressive solvents. The lifetime of these inks is sometimes very limited because of the degradation caused by exposure to ultraviolet light from sunlight. Thus, there is a need in the art for thermochromic systems in inks and coatings having resistance to degradation from exposure to ultraviolet light.
Temperature changes in thermochromic systems are associated with color changes. If this change is plotted on a graph having axes of temperature and color, the curves do not align and are offset between the heating cycle and the cooling cycle. The entire color versus temperature curve has the form of a loop. See generally FIG. 1A where the extent of color change presents a gap 100a that differs between color change that occurs upon heating 102 versus cooing 103. FIG. 1B presents a relatively larger gap 100b. Such a result shows that the color of a thermochromic system does not depend only on temperature, but also on the thermal history, i.e. whether the particular color was reached during heating or during cooling. This phenomenon is generally referred to as a hysteresis cycle and specifically referred to herein as color hysteresis or the hysteresis window. Decreasing the width of this hysteresis window to approximately zero would allow for a single value for the full color point and a single value for the clearing point. This would allow for a reliable color transition to be observed regardless of whether the system is being heated or cooled. Nonetheless, the concept of decreasing separation across the hysteresis window is elusive in practice. The extent of the respective gaps 100a, 100b to produce controlled hysteresis may be practiced according to the instrumentalities described herein.
Prior art reveals that the color transition range of microencapsulated thermochromic systems may be adjusted by shifting the full color point upward toward the clearing point, or shifting the clearing point downward toward the full color point, as explained in U.S. Pat. No. 4,028,118 issued to Norikazu et al. See also EP0480162 to Masayasu et al. These shifts are accomplished by adding high melting point materials to increase the full color point or, alternatively, by adding low melting point materials to the system to decrease the clearing point. Thus, the full color point or clearing point may be lowered or raised, but the overall temperature range between the two points remains unchanged because the amount of separation or width across the hysteresis window is left largely unaffected.
In recent years, metal decoration inks have been adapted for use or thermochromic pigments in high speed commercial canning operations. In one example of this, a thermochromic pigment may be formulated to use melamine formaldehyde microcapsules having an average diameter from 3 to 5 microns. This is poorly suited for use as a metal decoration ink for high speed application to a metal can where the line speed of the can coater may be greater than 1000 or 2000 cans per minute. The use of thermochromic metal decorating ink increasingly becomes a limiting factor at higher production line speeds. Problems arise in the ink rheology with this particle size that leads to misting as the ink is transferred at very high speed.
Presently, the use of thermochromic pigments in inkjet inks is not possible because creating particles sizes below one micron has not been possible. The larger particles interfere with the inks in the intended environment of use.